The invention relates to a compressor system for the compressed air supply of a commercial vehicle, the compressor system comprising a compressor for compressed air generation, a transmission for the transfer of torque from an engine to the compressor, a clutch for separating the connection between the compressor and the engine, and a control unit for opening and closing the clutch.
The invention relates, furthermore, to a method for controlling a compressor system, driven by an engine of a commercial vehicle, for the compressed air supply of the commercial vehicle, the compressor system comprising a compressor for compressed air generation, a transmission for the transfer of torque from the engine to the compressor, a clutch for separating the connection between the compressor and the engine, and a control unit for opening and closing the clutch.
So that the compressed air demand of subsystems of modern commercial vehicles can be satisfied, the compressors belonging to a compressed air supply device are equipped with a falling step-up ratio. The step-up ratio i between the rotational speed of the drive shaft ndrive, which is identical to the engine rotational speed, and the rotational speed of the output shaft noutput, which is identical to the compressor rotational speed, is in this case defined as:
  i  =            n      drive              n      output      and is selected so as to be as low as possible.
This is advantageous, since, particularly in the case of low engine rotational speeds, there is often an increased demand for air by the commercial vehicle. This is the case, for example, in container-changing operation or with regard to a bus which approaches a stop. The latter has to stop there, open the doors, first deaerate the air suspension, in order to make it possible to climb out easily if the floor level of the bus is low, close the doors again and then aerate the air suspension again before starting up. All the actions mentioned consume a large amount of air which has to be applied when the engine rotational speed is low.
As a result of the falling step-up ratio, however, the air quantity conveyed is increased greatly not only at low, but also at high engine rotational speeds, with the result that extreme loads upon the compressor may occur. This applies particularly since the nominal rotational speed of the engine of approximately 2000 revolutions per minute may be markedly overshot by the braking rotational speed (˜2400 rev/min) in the event of engine braking. The problem, here, is that, when engine braking is carried out, it is not desirable to cut off the compressor by use of the clutch in order to protect it, since air is possibly required for the service brake for braking the vehicle. This air consumption is even to be assumed since the commercial vehicle is already to be braked by the engine brake.
The object on which the invention is based is to provide a compressor system which can be operated with a step-up ratio markedly lower than one, while, in particular, the thermal and mechanical load occurring on the compressor is reduced at high engine rotational speeds.
This object is achieved by a compressor system for the compressed air supply of a commercial vehicle, the compressor system comprising a compressor for compressed air generation, a transmission for the transfer of torque from an engine to the compressor, a clutch for separating the connection between the compressor and the engine, and a control unit for opening and closing the clutch. The control unit is suitable for opening the clutch when an adjustable maximum permitted compressor rotational speed or engine rotational speed of the engine is overshot and for temporarily closing the open clutch while the engine rotational speed is higher than the maximum permitted engine rotational speed.
The invention provides a control unit that is suitable for opening the clutch when an adjustable maximum permitted compressor rotational speed or engine rotational speed of the engine is overshot and for temporarily closing the open clutch while the engine rotational speed is higher than the maximum permitted engine rotational speed. By the clutch being opened, the mechanical and thermal load upon the compressor when an adjustable maximum permitted rotational speed is overshot is first reduced to zero. By the clutch subsequently being closed temporarily while the engine rotational speed is higher than the maximum permitted engine rotational speed, on the one hand, the compressed air generation required is maintained and, on the other hand, the mean thermal and mechanical load upon the compressor is reduced, as compared with the continuous operation. Typical step-up ratios at which the use of a compressor system according to the invention is especially preferred lie between 0.4 and 0.7, although the compressor according to the invention can be used, in general, when low step-up ratios or high nominal rotational speeds of the engine result when compressor rotational speeds are high.
Expediently, there may also be provision for the control unit to be suitable for opening the clutch when a pressure present in the compressed air supply system of the commercial vehicle overshoots an adjustable cut-off pressure. If the storage containers for compressed air of the commercial vehicle are sufficiently full, a further generation of compressed air by the compressor is unnecessary. The pressure prevailing in the compressed air supply system of the commercial vehicle may be adopted as a simple indicator of the degree of filling.
Advantageously, there may also be provision for the control unit to be suitable for opening the clutch while the engine is being started. By the clutch being opened while the engine is being started, the torque to be applied by a starter in order to start the engine can be reduced.
Furthermore, there may be provision for a temperature sensor to be provided, which detects the temperature of the compressor. The temperature of the compressor may be adopted as a measure of the thermal load upon the compressor.
In particular, there may be provision for the transmission to have a step-up lower than one. By a step-up lower than one being used, the compressor rotational speed rises in relation to the engine rotational speed, and consequently the volume of the compressed air generated rises in the case of a specific rotational speed of the engine. Thus, particularly in the case of low engine rotational speeds, the compressed air quantity required can be provided quickly.
The invention relates, furthermore, to a commercial vehicle having a compressor system according to the invention.
According to the invention, the clutch is opened when an adjustable maximum permitted compressor rotational speed or engine rotational speed of the engine is overshot, and an open clutch is temporarily closed while the engine rotational speed is higher than the maximum permitted engine rotational speed.
Thus, the advantages and particular features of the compressor system according to the invention are also implemented within the framework of a method. This also applies to the particularly preferred embodiments, given below, of the method according to the invention.
The method is expediently developed in that the clutch is opened when a pressure present in the compressed air supply system of the commercial vehicle overshoots an adjustable cut-off pressure. Furthermore, there may be provision for the clutch to be opened while the engine is being started. There may also be provision for the clutch to be opened when a measured temperature of the compressor overshoots an adjustable maximum permitted temperature. In this regard, there may be provision, furthermore, for the clutch to be temporarily closed while a measured temperature of the compressor overshoots an adjustable maximum permitted temperature.
In particular, there may be provision for the transmission to transfer the engine rotational speed to the compressor with a step-up lower than one.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.